1. Field of the Invention
The present invention relates to a turbocharger in which a back plate is disposed between a compressor housing and a bearing housing, and the invention also relates to a method of producing the turbocharger.
2. Description of the Related Art
A turbocharger installed in a car or the like is configured such that air sucked in a compressor is compressed and discharged toward an internal combustion engine. That is, a discharge scroll chamber into which compressed air discharged from an impeller flows is formed in an air flow passage formed inside of a compressor housing, the discharge scroll chamber guides compressed air into a discharge port, and the compressed air from the discharge port is discharged toward the internal combustion engine. Especially, a shape of the discharge scroll chamber largely influences performance of the compressor, and it is required to finish the discharge scroll chamber into an appropriate shape in accordance with required performance.
Here, there is a method of producing the compressor housing by gravity die casting for example. In this case, since casting can be carried out using a so-called core, flexibility in shape is high and a complicated shape can be formed. However, since a casting cycle is long, productivity is poor and costs are high. Further, if a sand mold is used, a surface roughness becomes large and thus, there is a problem that efficiency of the compressor is deteriorated.
There is also a method of forming the compressor housing by die casting. In this case, since the casting cycle is short as compared with the gravity die casting, the productivity is superior and costs are low. However, since the compressor housing can be formed only when a molded shape can be extracted from a mold and thus, flexibility in shape is poor and a complicated shape can not be formed. Hence, there is a compressor housing formed by assembling three pieces, i.e., a scroll piece, a shroud piece and an outer peripheral annular piece as disclosed in Patent Document 1. According to this, the pieces are formed into shapes which can easily be formed by die casting, and flexibility in shape of a discharge scroll chamber of the compressor housing is secured.
In a turbocharger described in Patent Document 2, a compressor housing is composed of two pieces, i.e., a scroll piece and a shroud piece. An outer periphery of a back plate disposed on a side opposite to a suction side in the compressor housing is provided with a curved surface, and this curved surface is formed as a part of an inner wall surface of a discharge scroll chamber.